I always thought a smile to yourself was a gesture of ridiculousness or something not positive, The last time I did it was to a girl that denied me online that I happen to see in person, and before that it was to a girl who was ignoring me because I never called her back, and if you don't have history with the guy he could be thinking anything. I hate to keep throwing out examples but it could mean you look too easy or he thinks he can do better. I wouldn't want a girl to do that